Detection and/or monitoring of certain cardiac disorders and diseases require long term intermittent ECG monitoring. Irregular heart beat, for example, is a group of disorders that are often not persistent or chronic, and thus often not present when a resting 12-lead ECG is obtained. This has prompted the development of easy-to-use long-term ECG recording devices, which typically rely on a single lead ECG or a small number of leads. These include electrode patches that can be continuously worn for weeks at a time or simple hand-held ECG devices that can be used by the patient to periodically (yet regularly) capture 30 seconds of ECG data.
Atrial fibrillation (“AFIB”) is the most common form of an irregular heartbeat. In AFIB, the heart's atrial walls do not produce an organized contraction and instead quiver. Thus, no organized heart beat rhythm can be detected from a patient experiencing AFIB. This arrhythmia puts patients at significant risk because it allows blood to pool and stagnate in the left atrium and, thus, form a clot. This clot can slough off and travel up to the brain where it can block sufficient blood flow to a portion of the brain where upon it will begin to die, thus causing a stroke. AFIB causes up to a quarter of all strokes and is often undetected until a stroke occurs. It is estimated that approximately a third of AFIB is asymptomatic. The prevalence of AFIB is high and age dependent, from 0.7% in the ages 55-59 to 17.8% for 85 years or older. Yet, AFIB is notoriously hard to detect.
Intermittent AFIB has been found to increase the risk of stroke, although perhaps to a lesser degree than persistent AFIB. Moreover, it has been found that a significant portion of those who suffer from transient ischemic attacks or cryptogenic strokes have intermittent AFIB. Thus, there is increased interest in recording transient episodes of AFIB which requires something other than the use of a diagnostic resting ECG device. However, detecting AFIB in an ECG recording poses challenges.
The most common method for automatically detecting AFIB in ECG recordings relies heavily on the fact that AFIB is a chaotic atrial arrhythmia, randomly conducted to the ventricles. As such, the time periods between features of the QRS waves, as measured by the RR intervals, should be continuously varying in the presence of AFIB. It is this attribute of AFIB, that it is a continuously chaotic rhythm, that is used by most ambulatory ECG analysis programs to detect AFIB. However, RR intervals of normally conducted beats can vary for other types of benign arrhythmias that are not AFIB. Examples include premature atrial complexes (PACs) or sinus arrhythmia (SA), which are both quite common in the normal population. While these benign arrhythmias do exhibit as variability in RR intervals, these arrhythmias are not actually continuously chaotic or random. Differentiating between benign arrhythmias and AFIB requires specialized training and is time consuming.